In a wireless transmission system that performs wireless transmission based on a W-CDMA system, an uplink signal, that is transmitted from mobile stations (UE: User Equipment) located within the same cell, is multiplied with a user-specific scramble code, and the received signal of the uplink signal is non-orthogonal between the UEs located within the same cell. Therefore, high-speed transmission power control (TPC) has become essential in order to reduce the influence of multi-user interference (i.e., the near-far problem).
Whereas, in the LTE (Long Term Evolution) system specified in the 3GPP Release 8 (hereinafter referred as “Rel-8 LTE”), an SC-FDMA (Single-Carrier Frequency Division Multiple Access) system, which is effective for achieving a low peak-to-average power ratio (PAPR: Peak-To-Average Power Ratio) and increases coverage, employs an uplink transmission. The SC-FDMA system basically allocates a wireless resource configured from a predetermined frequency resource and a predetermined time resource to one UE via scheduling by the base station. Therefore, intra-cell orthogonality is achieved among a plurality of users at a frequency domain and time domain. Accordingly, from a viewpoint of suppressing intra-cell multi-user interference, TPC is not necessarily essential. However, in the Rel-8 LTE, since all of the cells are based on a one-cell frequency reuse, that uses the same frequency, a large amount of intra-cell interference from peripheral cells occurs, and the level of inference that cell-edge UEs receive from UEs of other cells is particularly high. Therefore, in order to compensate such peripheral cell interference and maintain a constant reception quality, it is necessary to apply TPC also to LTE.
In the Rel-8 LTE uplink, a 1) physical random access Channel (PRACH), a 2) physical uplink shared channel (PUSCH), and a 3) physical uplink control channel (PUCCH) are specified.
Only a low-speed TPC (open-loop TPC), which compensates for distance attenuation and shadowing variations, is applied to the PRACH.
PUSCH is a physical channel for transmitting user data. The adaptive modulation and channel coding (AMC) in accordance with the reception channel state of the UE and an adaptive TPC are applied to the PUSCH. In this case, compensation of pass loss and shadowing variations are dealt with by using an appropriate TPC (open-loop TPC), and instantaneous fading variations are dealt with by an adaptive rate control via the AMC.
On the other hand, PUCCH is a physical channel for transmitting control information (typically downlink reception channel quality (CQI: Channel Quality Indicator) information and downlink acknowledgement (ACK)/negative ACK (NACK), etc.). Since the transmission bit number of such control information is predetermined, the transmission rate thereof is fixed, and hence does not require an adaptive rate control via the AMC in accordance with the reception channel quality. Rather, it is important for the information transmitted by the PUCCH to maintain a constant reception quality due to the essential requirement of feedback for the AMC or for repeating that is applied in the downlink data channel. Therefore, with respect to the PUCCH, since it is necessary to compensate the reception level, including pass loss, shadowing variations, and instantaneous fading variations, a closed-loop TPC should be applied in addition to the open-loop TPC.
FIG. 2 is a configuration diagram of the PUCCH in the Rel-8 LTE uplink.
In order for the PUCCH to be both always transmittable and have a low overhead, a narrow bandwidth (180 kHz) wireless resource is employed. Furthermore, a 1 msec subframe is configured from two 0.5 msec slots. In the case of using a narrow transmission bandwidth, generally the frequency diversity gain is reduced. However, by utilizing the two slots within one subframe and applying frequency hopping between the bandwidths at each end of the transmission spectrum, a large frequency diversity effect can be obtained. Transmission bandwidths RB1 and RB2 are allotted in the bandwidths at each end of the frequency hopped transmission spectrum, frequency hopping is executed between slot 1 of RB1 and slot 2 of RB2 for UE1, and frequency hopping is executed frequency hopped between slot 1 of RB2 and slot 2 of RB1 for UE2. The transmission bandwidths RB1 and RB2 can also be referred to as resource blocks (RB). Furthermore, a plurality of mobile stations, to which frequency hopping is applied, that use the same wireless resource are orthogonalized by code division multiplexing.
FIG. 5 is a conceptual diagram of the closed-loop TPC with respect to the PUCCH specified in the Rel-8 LTE. As shown in FIG. 5, in the Rel-8 LTE uplink, a mode that periodically (TCQI) transmits the CQI information in the PUCCH. In this case, the uplink reception channel quality (SINR) is measured in the base station using a reference signal (RS), for estimating the channel, that is sent by the PUCCH. The reception SINR and the target reception level are compared, and a TPC command bit is generated in order to control the transmission power so as to have a constant reception quality. For example, the base station generates a TPC command bit at a given frequency (TTPC) and transmits it to a mobile station. At the mobile station, the transmission power of the PUCCH is adjusted in accordance with the received TPC command bit. The uplink closed-loop TPC can be achieved in this manner.
FIG. 6 is a conceptual diagram of a TPC method (referred to in this specification as “RB common TPC method”) with respect to the PUCCH in the Rel-8 LTE. In the RB common TPC method, the reception SINR for each of the frequency hopped RB1 and RB2 of the PUCCH is measured, and the average reception SINR is obtained by averaging the measured reception SINR between the RB1 and the RB2. A TPC command bit is sent to the UE so that the average reception SINR is made to equal the target SINR value, and closed-loop control is performed.
The RB common TPC method is a method for performing common power control between the RBs with respect to the UE in order to make the average reception SINR, of the two RB1 and RB2 that used for frequency hopping, become equal with the target reception SINR value. As shown in FIG. 6, at the transmission end (Tx), the RB1 and RB2 are adjusted to have a common (same) power, since frequency selective fading occurs in the channel propagation path between the mobile station and the base station, the reception SINRs at the base station have differing reception levels between the RB1 and the RB2. In the example shown in FIG. 6 indicates a state in which the average reception SINR of the RB1 and the RB2 is equal to the target SINR; however, the RB1 is received at an excessive quality that exceeds the target value, whereas, the RB2 is received at a quality that is under the target value.